Probably a mistake... according to Bulbapedia, it has 100% accuracy but of course can only be used on sleeping Pokemon. It's supposed to give the user half of the damage dealt, back as HP. So if Dream Eater deals... 30 damage, the user gets 15 back in HP points.

"Dream Eater only works if the target is asleep. Dream Eater inflicts damage and 50% of the damage dealt is restored to the user as HP. If this attack deals 1 damage, 1 HP will be restored to the user."